1. Field of the Invention
The present invention concerns a magnetic resonance (MR) system for generation of magnetic resonance exposures of an examination subject in a patient positioning region. The magnetic resonance system is of the type having an antenna structure with a number of antenna elements arranged in the patient positioning region feed lines that respectively supply the antenna elements with radio-frequency signals for emission of a radio-frequency field in the patient positioning region and/or to accept radio-frequency signals acquired by the antenna elements; and a radio-frequency shielding that shields an external region outside of the patient positioning region from radio-frequency signals radiated in the patient positioning region.
2. Description of the Prior Art
Magnetic resonance tomography is a widely used technique for acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this method, the body or the body part of the patient or examination subject must initially be exposed to an optimally homogeneous static basic magnetic field which is generated by a basic field magnet of the magnetic resonance system. Rapidly switched gradient fields for spatial coding that are generated by gradient coils are superimposed on this basic magnetic field during the acquisition of the magnetic resonance images. Moreover, radio-frequency pulses of a defined field strength are radiated into the examination subject with radio-frequency antennas. The nuclear spins of the atoms in the examination subject are excited by means of these radio-frequency pulses such that they are deflected from their equilibrium state parallel to the basic magnetic field by “excitation flip angle”. The nuclear spins then precess around the direction of the basic magnetic field. The magnetic resonance signals generated upon relaxation in the initial position are acquired by radio-frequency reception antennas. The magnetic resonance images of the examination subject are generated on the basis of the acquired magnetic resonance signals.
A typical magnetic resonance tomography apparatus has a patient positioning region (also called a patient space in the following) in which is located a patient bed on which the patient is positioned during the examination. For example, this can be a “patient tunnel” in a tube running through the housing of the tomography apparatus. Moreover, hoe are also MR tomograph apparatuses with a laterally open patient positioning region that is enclosed in a U-shape structure formed by the housing of the MR tomography. A number of coils and possibly also permanent magnets for generation of the necessary basic magnetic field and the gradient fields are typically located within the housing of the tomography apparatus.
Furthermore, the tomography apparatus typically has an antenna structure permanently installed in the housing, with which antenna structure the necessary radio-frequency pulses are emitted into the patient positioning region and the induced magnetic resonance signals can be acquired. This radio-frequency antenna is also known as a “body coil”. Such a body coil, for example, frequently has a used birdcage structure composed of a number of conductor rods arranged around the patient space and running parallel to the primary field direction. The conductor rods are connected with one another by annular conductors at the front sides of the coils. Other structures such as, for example, saddle coils are known. In order to shield the external region outside of the patient positioning region from the B1 field (i.e. the radio-frequency field) that is generated by the antenna structure and in order to minimize interference from the external region during MR signal acquisition, the patient positioning region is typically surrounded by a radio-frequency shielding normally at ground potential. For example, the shielding can be thin copper layers or the like. Since, as already described in the preceding, the tomography apparatus housing normally extends annularly or in some embodiments in a U-shape around the patient positioning region, this radio-frequency shielding either annularly or partially encloses the patient space.
In conventional MR systems the feed of the radio-frequency antenna with radio-frequency signals has previously normally ensued via two feed lines that are directed in the longitudinal direction of the patient positioning region within the shielded patient positioning region and, for example, outward to radio-frequency circuit arrangements at the front side of the apparatus. The radio-frequency circuit arrangements typically have radio-frequency power amplifiers and possibly further circuit components in order to control the antenna structure and to monitor and/or pre-process signals acquired by the antenna structure.
Moreover, in addition to these permanently installed antenna structures there are also local coils that are arranged optimally close to the patient or subject, i.e. are moved with the patient into the patient positioning region.
An example of such a local coil is a head coil as disclosed in U.S. Pat. No. 5,483,163. The coil described there is specifically designed as a small birdcage structure, and the antenna elements running in the longitudinal direction are fashioned not as rods but rather as individually pivotable conductor loops in order to be able to detune the head coil.
Further examples for local coils are provided in WO 2005/012931 A1. Among other things, a surface coil is described therein for placement on or to be placed under a patient, this surface coil having an array of individual conductor loops. For inductive decoupling the conductor loops are shaped in specific geometries and arranged in a specific manner relative to one another, so as to overlap.
In principle local coils, can be used both for transmission of the excitation pulse sequences and for acquisition of the magnetic resonance signals. Due to the smaller distance from the examination subject, they normally have a better reception quality than the permanently installed body coil. In most cases the body coil is therefore used to emit the excitation pulse sequences and the local coils serve to acquire the magnetic resonance signals. In such a method the local coils must be deactivated during the transmission procedure and the body coil must be activated. In reverse, upon acquisition the local coils must be activated and the body coil must be deactivated. The deactivation of a coil can ensue by sufficiently detuning it relative to the magnetic resonance frequency. For this purpose, the coils are equipped with switching devices. Such a switching device is described in DE 10 2006 019 173.
In order to be able to influence the structure of the radiated magnetic field with optimal detail in a suitable manner in all regions of the examination volume, in order to achieve an optimally good homogeneity of the B1 field in the examination volume, the trend of future developments in the field of magnetic resonance systems is moving toward using a number of separately-controllable antenna elements for emission of the radio-frequency signals instead of a simple antenna structure that can be controlled via only two feed lines. An example of this is described in DE 101 24 465 A 1, which discloses an arrangement for generation of radio-frequency fields in the examination volume of an MR apparatus that has a number of separately-controllable resonator segments (i.e. antenna elements). The resonator segments are arranged in a birdcage antenna around the patient space and are respectively formed by at least one conductor element running in parallel.
The use of a number of separately controllable antenna elements simultaneously entails an increase of the number of feed lines to the antenna elements. The previously typical direction of the feed lines (usually executed in the form of coaxial lines) to the antenna elements in the longitudinal direction in the patient space is therefore disadvantageous for multiple reasons. Space problems arise due to the increase of the feed lines to the antenna elements within the patient space. In part it is necessary that the feed lines intersect, and this can lead to interference due to crosstalk. Moreover, asymmetrical currents that can influence the magnetic resonance acquisitions in an unwanted manner can typically occur on the outer conductors of the feed lines due to this manner of the direction and the length of the feed lines. At least the latter cited problem was previously solved by the use of sheath wave barriers (known as baluns) with high impedances that prevent the propagation of unwanted waves on the external conductors. However, this incurs additional costs for the installation of the sheath wave barriers and requires additional space.